The present invention relates generally to implantable cardiac therapy devices, and more particularly, to a method and system for facilitating two-sided telemetry in implantable cardiac therapy devices.
Implantable cardiac therapy devices include implantable pacemakers and implantable cardioverter-defibrillators. An implantable pacemaker monitors the intrinsic electrical activity of the patient's heart and if a natural heart beat is not detected within a prescribed time period, the pacemaker delivers (via a lead system) an electrical stimulation or pacing pulse to force the heart muscle tissue to contract, thereby assuring that a minimum heart rate is maintained. In this way, bradycardia is terminated or prevented. Contemporary implantable cardioverter-defibrillators (ICDs) monitor the intrinsic electrical activity of the patient's heart in accordance with a diagnostic or detection algorithm by analyzing electrograms (EGMs) generated by sensing electrodes positioned proximate the sino-atrial and/or atrio-ventricular node of the patient's heart, and most advantageously, in the right ventricular apex of the patient's heart.
Typical current-generation ICDs are capable of delivering various types or levels of cardiac therapy (i.e., "tiered therapy"). The first type or level of therapy is bradycardia and antitachycardia pacing (ATP), in which a low level of electrical energy (generally between millionths to thousandths of a joule) is delivered to the patient's heart (via a lead system) in order to correct detected episodes of bradycardia or tachycardia, respectively. The second type or level of therapy is cardioversion, in which an intermediate level of electrical energy (generally between 1-5 joules) is delivered to the patient's heart (via a lead system) to terminate a detected episode of ventricular arrhythmia (e.g., a detected heart beat in the range of 130-190 beats/minute) or an ongoing episode of tachycardia that ATP therapy has failed to terminate. The third type or level of therapy is defibrillation, in which a high level of electrical energy (generally above 15 joules) is delivered to the patient's heart (via a lead system) in order to terminate a detected episode of ventricular fibrillation or an episode of ventricular tachycardia which has degraded into ventricular fibrillation due to failure of cardioversion therapy. The defibrillation energy is typically stored in a defibrillation energy storage capacitor ("output capacitor") which is charged by a high-voltage charging circuit, and then delivered as an electrical shock(s) by means of a high-voltage output switching circuit which discharges the output capacitor.
Current generation ICDs are microprocessor-controlled. The detection and diagnosis of cardiac arrhythmias which require treatment are performed in accordance with a complex detection or diagnostic algorithm programmed into the microprocessor, and the delivery of the appropriate form of cardiac therapy is controlled in accordance with an equally complex therapy delivery algorithm programmed into the microprocessor. The goal is to optimize the therapy for a given patient using the lowest amount of energy which is possible (for a given safety margin), thereby enabling the ICD to be made as small as possible.
Current generation ICDs store electrogram, device operating status (e.g., battery status and lead impedance), and diagnostic data in device memory (e.g., RAM). An external data telemetry device (sometimes referred to as a "programming wand") is employed by the patient's physician to telemetrically read-out the stored data in a non-invasive manner, with little or no discomfort to the patient. The patient's physician can analyze the telemetered data in order to evaluate the status of the device and the therapeutic efficacy thereof. The telemetered data can aid the physician in gaining a better understanding of the etiology of the patient's underlying cardiac condition, and enable the physician to better "customize" the therapy administered by the ICD for the patient by telemetrically reprogramming the device using the programming wand.
As can be appreciated from the foregoing, the capability of telemetrically interrogating and programming the ICD using an external, programming wand is of critical importance. Generally, the programming wand-ICD telemetry interface includes separate transmit and receive coils in the programming wand and a single transmit/receive (T/R) coil in the ICD. A typical programming wand is disclosed in U.S. Pat. No. 4,809,697 and typical telemetry circuitry is disclosed in U.S. Pat. No. 4,944,299, the disclosures of which are incorporated herein by reference.
Data (such as device status, diagnostic, and electrogram data) is telemetrically read-out of the ICD by flowing a current through the T/R coil in a fixed direction, thereby producing a magnetic field having a fixed direction. The magnetic field produced by the current flowing through the T/R coil of the ICD induces a corresponding current in the receive coil of the external programming wand, when the programming wand is held in close proximity to the patient's chest at the location where the ICD is implanted. The current is RF-modulated in accordance with the data being transmitted. Thus, the RF-modulated current constitutes an information signal which can be converted by an A/D converter in the programming wand and then decoded by suitable decoding circuitry in a customized desktop or laptop personal computer to which the programming wand is coupled, to thereby reconstitute the original data for presentation to the physician (typically on a display screen of the customized personal computer).
Data (such as command data for re-programming the ICD) is telemetrically programmed into the ICD by RF- modulating current flowing through the transmit coil of the programming wand in a fixed direction, thereby producing a magnetic field having a fixed direction. The magnetic field produced by the current flowing through the transmit coil of the programming wand induces a corresponding current in the T/R coil of the ICD, when the programming wand is held in close proximity to the patient's chest at the location where the ICD is implanted. Again, the current is RF-modulated in accordance with the data being transmitted. Thus, the current constitutes an information signal which can be converted by an A/D converter in the ICD and then decoded by suitable decoding circuitry in the ICD, to thereby reconstitute the original data for processing by the microprocessor and/or other appropriate logic hardware within the ICD.
Although the above-described ICD-programming wand telemetry interface has proven quite adequate for most situations, it does suffer from at least the following significant shortcoming. Namely, successful telemetry using the presently available programming wands requires a "correct" orientation of the programming wand and ICD. The "correct" orientation is the orientation in which the fixed direction of the magnetic field produced by the programming wand induces current flow through the T/R coil of the ICD in the fixed direction for which the telemetry circuitry in the ICD is designed to detect current flow through the T/R coil. The "correct" orientation is depicted in FIG. 1. However, if the ICD is flipped so that the device and the programming wand are not in the "correct" orientation, telecommunication (telemetry) between the programming wand and the ICD is not possible. This situation is depicted in FIG. 2. In this connection, an ICD can be inadvertently flipped as a result of a patient's twiddling with the device by manipulating the skin of his or her chest where the device is implanted, a condition which has been dubbed "twiddler's syndrome".
Another shortcoming of the existing programming wand-ICD telemetry interface is that the device must be implanted with the "correct" side up in order to facilitate "correct" orientation of the programming wand and the device. Thus, the leads that are connected to the device must trail out of one side of the device, e.g., the right side, even though it may be desired that the leads trail out of the other side of the device, e.g., the left side. In short, the requirement of a single "correct" orientation (or, "one-sidedeness" of the wand-ICD telemetry interface) acts as a constraint on the implantation of the device.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a programming wand-ICD telemetry interface which overcomes the above-described shortcomings of the presently available technology. The present invention fulfills this need in the art.